Method of treating aluminum



Patented July 30, 1940 UNITED STATES] PATENT OFFICE N Drawing.

Claims.

- This invention relates to methods of treatingaluminum and particularlyto methods of preparing aluminum for use in electrolytic devices such aselectrolytic condensers, rectifiers, lightning arresters and the like.This application is a continuation in part of the copending applicationsof Joseph B. Brennan Serial No. 707,549, filed...

January 20, 1934, and of Joseph B. Brennan and Leona E. Marsh, SerialNo. 118,839, filed January 2, 1937.

In the copending applications referred to above there are disclosedmethods of preparing aluminum particularly for use in electrolyticcondensers in which the surface of the aluminum is electrolyticallytreated in order to clean and/or etch the surface of the aluminum. Thepresent application relates especially to the cleaning and/or etching ofaluminum in a solution of aluminum chloride (A1013). It is among theobjects of my invention to provide a method of cleaning and/ or etchingaluminum which can be accurately and easily controlled to produceuniform results. Another object is to provide a method of cleaning andetching aluminum which can be carried out i at extremely low cost. Afurther object is to provide a method of treating aluminum whereby thesurface may be thoroughly cleaned and etched or roughened in such amanner asto produce a large increase in the superficial area thereof.According to my invention the surface ofaluminum is cleaned and etchedby subjecting it to electrolysis in an electrolyte consistingprincipally of a solution of aluminum chloride (AlCla). The 1 withoutany preliminary cleaning being required,

is immersed in an aqueous solution of aluminum 5 chloride and subjectedto electrolytic action as an anode. The cathode preferably is composedof an inert material such as carbon, and the method may .be carried outin a simple apparatus, as will be readily appreciated by those skilledin the art. 0 The aluminum anode and the inert cathode are provided withterminals or leads, which may be connected to positive and negativeterminals, respectively, of any convenient source of direct cur rent.The flow of current results in a thorough 5 cleaning and etching of thesurface of the alumi- Application May 6, 1937, Serial No; 141,144

.num which appears to have the effect of greatly increasing thesuperficial area thereof.

In carrying out the electrolysis, I find that neither the strength ofthe solution, the voltage applied, nor the time of treatment need beheld 5 within close ranges. Thus I preferably employ a 3to 5% by weightaqueous solution of alumi num chloride, voltages may vary from 3 to 20volts, and the time of treatment vary from 5 to 15 minutes. I have foundthat very satisfactory resultscan be obtained by employing a 5% solutionof aluminum chloride, a voltage of about 6 to 10 volts, and a current ofabout 7 amperes in the case of a piece of aluminum foil measuring 3 in.x 3 in. The current is caused to flow for about 15 minutes, and I findthat when a piece of aluminum foil'so treated is pr vided with adielectric film and incorporated in an electrolytic condenser, thecapacity is increased from 4 to 5 times ascompared with an untreatedpiece-of similar foil incorporated in a similar condenser. Thisindicates that probably the superficial area of the aluminum isincreased from 4 to 5 times by the etching operation. l After subjectingthe aluminum foil to electro-- lysis as described, the aluminum isremoved from the aluminum chloride solution and the electro-' lyteremoved therefrom by washing either in water according to conventionalmethods or preferably by exposure to steam as disclosed in the copendingapplication of Brennan and Marsh referred to above.

After Washing, the aluminum [is ready for the film forming operation ifintended for use as-a condenser anode. The dielectric film may be formedaccording to any well known method. For example, aluminumnmy be anodizedin abath of boric acid or boric acid andammonium borate, depending uponthe service for which the condenser is intended. The forming voltagesmay be controlled in the usual manner, and in the case of high voltagecondensers the operation is carried on until the leakage in the formingbath at 550 volts and a temperature of 90 .C. is, about 0.1 to 1milliampere per microfarad of capacity. This leakage is further reducedwhen the anode is assembled in a condenser at room temperature. Theforming voltages and the method used in forming the dielectric film maybe varied depending upon the service for which the condenser isintended, as will be evident to those skilled in the art.

The etching electrolyte may be prepared either by dissolvingaluminumchloride directly in water which produces a slightly acidelectrolyte beu I cause of the hydrolysis-of the aluminum chloride, orif desired the electrolysis can be started with a 1% to 5% solution ofhydrochloric acid. In the latter case, the solution soon has the samecharacteristics as the solution made by dissolving A1013 in water forthe acidity of the solution rapidly decreases as the reactions proceedand aluminum chloride is formed as the result of the action of thehydrochloric acid on the aluminum.

The solution remains slightly acid because of hydrolysis which acidityremains substantially For example, the etching-method may be applied tothe production of cathodes for condensers or to etching or cleaningaluminum for decorative purposes.

Various other treatments can be given the aluminum both before and afterthe electrolytic cleaning and etching step and various othermodifications and changes in the invention, all within the spirit andscope thereof, will be evident to those skilled in the art. Thereforeit/is to be understood that my patent is not limited to the specificdisclosure contained herein or in any manner other than by the scopeofthe appended claims when given the range of equivalents to which mypatent may be entitled.

I claim:

1. The method of etching aluminum 1 which comprises connecting thealuminum to an external source of electromotive force and subjecting thealuminum to electrolysis as an anode in an electrolyte consistingprincipally of an aqueous solution of aluminum chloride.

2. The method of preparing aluminum for use as an electrode in an'electrolytic cell which includes the steps of connecting the aluminum toan external source of electromotive force and subjecting the aluminum toelectrolysis as an anode in an electrolyte consisting principally of anaqueous solution of aluminum chloride until the surface presents anetched or pitted appearance.

3. A method of preparing aluminum for the reception of a dielectric filmincluding the steps of immersing the aluminum in an electrolyteconsisting principally of an aqueous solution of aluminum chloride,connecting the aluminum to an external electric circuit, causing anelectric current to flow through the electrolyte, the aluminum being theanode, and thereby causing the surface of the aluminum to be etched, andcontrolling the etching reaction by controlling the flow of current.

4. A method of etching aluminum which comprises connecting the aluminumto an external source of electromotive force and subjecting the ofelectromotive force and subjectingaluminunr to electrolysis as an anodein an electrolyte consisting principally of an aqueous solution ofaluminum chloride until the efieotive surface area.

of the aluminum is increased from 4 to 5 times.

JOSEPH B. BRENNAN.

